The present invention relates to a silicon carbide reinforced composite material having a combination of a high fracture toughness with a high mechanical strength. More particularly, the present invention is concerned with a silicon carbide reinforced alumina composite material and a silicon carbide reinforced silicon nitride composite material.
Ceramic materials, such as alumina and silicon nitride, have been widely used as various structural materials for use under high-temperature conditions by virtue of their excellent mechanical strength and thermal and chemical stabilities.
Alumina and silicon nitride, however, have a disadvantage as a material in that the fracture toughness is poor. For this reason, in recent years, studies have been energetically conducted on an improvement in the fracture toughness. For example, various proposals have been made on a composite material comprising a ceramic base material and various whiskers.
The above-described improvement in the toughness of the ceramic derived from the compositing of the base material with a whisker has been thought to be attributable to the inhibition of the growth of crack occurring in the ceramic by virtue of the whisker dispersed in the sinter texture of the ceramic, the effect of increasing the critical strain energy release rate through the deviation of the direction of the progress of the crack by the whisker (the deflection effect), or the effect of prevent cracking by the whisker existing in the direction of progress of the crack (the pullout effect).
Although the incorporation of a large amount of a whisker in the ceramic base material is useful for improving the toughness of the base material, it is known that this lowers the strength of the base material [J. Am. Ceram. Soc., 72 (5), 791-798 (1989); and ibid., 65 (2), 351-356 (1986)].
This is because the incorporation of a large amount of the whisker makes it difficult to homogeneously disperse the whisker in the base material due to aggregation among the whiskers, so that smooth sintering of the composite material is apt to be inhibited.
In recent years, attention has been paid to a mechanism for improving the toughness by bridging the whisker, and it was reported that the compositing with a whisker having a large diameter was effective in improving the toughness [see The Ceramic Society of Japan "Proceeding of Fall Meeting of the Ceramic Society of Japan 1989", pp. 490-491]. The term "bridging" used herein is intended to mean such an effect that even after the crack has proceeded, the bonding between the whisker and the base material is held to lower the stress at the front end of the crack and also when the crack further proceeds and the whisker is pulled out, the stress of the front end of the crack is similarly lowered by frictional force between the whisker and the base material.
The present inventors proposed a process for producing a composite ceramic material having improved strength and toughness by compositing a ceramic base material with a silicon carbide particle having a mean particle diameter of 1 to 20 .mu.m and/or a silicon carbide whisker having a mean diameter of 1 to 10 .mu.m and an aspect ratio of 1 to 15 in a volume fraction (Vf) of 5 to 50% (see Japanese Patent Laid-Open No. 12066/1992). Further, it was reported that the toughness was improved by simultaneously incorporating a silicon carbide whisker and a silicon carbide particle into silicon nitride [J. Am. Ceram. Soc., 73 (3) 678-683 (1990)], though the strength was lower than that when only the silicon nitride was used. The present invention has been made based on a finding that the incorporation of an elliptical silicon carbide particle together with the silicon carbide whisker is effective in improving the toughness through the incorporation of the silicon carbide whisker while maintaining the strength inherent in the silicon nitride base material.